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What is a Sensor?
A sensor is a device that measures a physical quantity and converts it into a signal
which can be read by an observer or by an instrument.
For example, a thermocouple converts temperature to an output voltage which can be
read by a voltmeter.
For accuracy, all sensors need to be calibrated against known standards
How to choose a sensor?
Environment: There are many sensors that work well and predictably inside, but that
choke and die outdoors.
Range: Most sensors work best over a certain range of distances. If something comes
too close, they bottom out, and if something is too far, they cannot detect it.
Thus we must choose a sensor that will detect obstacles in the range we need.
Field of View: Depending upon what we are doing, we may want sensors that have a
wider cone of detection. A wider “field of view” will cause more
objects to be detected per sensor, but it also will give less information
about where exactly an object is when one is detected.
Types of Sensors
Thermal Energy Sensors
Optical and Radiation Sensors
What is an Image Sensor?
An image sensor is a device that converts an optical image into
an electrical signal.
Unlike traditional camera, that use film to capture and store an
image, digital cameras use solid-state device called image
Image sensors contain millions of photosensitive diodes known
as photo sites.
When you take a picture, the camera's shutter opens briefly and
each photo site on the image sensor records the brightness of the
light that falls on it by accumulating photons. The more light
that hits a photo site, the more photons it records.
What is a Pixel?
The smallest discrete component of an image or
picture on a CRT screen is known as a pixel.
Each pixel is a sample of an original image,
where more samples typically provide more-
accurate representations of the original.
What is Fill Factor?
Fill factor refers to the percentage of a photo
site that is sensitive to light.
If circuits cover 25% of each photo site, the
sensor is said to have a fill factor of 75%. The
higher the fill factor, the more sensitive the
Image Sensor History
Before 1960 mainly film photography was done and vacuum tubes were being used.
From 1960-1975 early research and development was done in the fields of CCD and
From 1975-1990 commercialization of CCD took place.
After 1990 re-emergence of CMOS took place and amorphous Si also came into the
Types of Image Sensors
CCD: Charged Coupled Device CMOS: Complementary Metal Oxide
Charged Coupled Device (CCD)
Charge-coupled devices (CCDs)
are silicon-based integrated
circuits consisting of a dense
matrix of photodiodes that
operate by converting light
energy in the form of photons
into an electronic charge.
Electrons generated by the
interaction of photons with
silicon atoms are stored in a
potential well and can
subsequently be transferred
across the chip through registers
and output to an amplifier.
Basic Operation of a CCD
In a CCD for capturing images, there is a
photoactive region, and a transmission region
made out of a shift register (the CCD, properly
An image is projected by a lens on the capacitor
array (the photoactive region), causing each
capacitor to accumulate an electric charge
proportional to the light intensity at that location.
A one-dimensional array, used in cameras,
captures a single slice of the image, while a two-
dimensional array, used in video and still
cameras, captures a two-dimensional picture
corresponding to the scene projected onto the
focal plane of the sensor.
Types of CCD Image Sensors
1. Interline Transfer CCD Image Sensor 2. Frame Transfer CCD Image Sensor
Interline Transfer vs. Frame Transfer
Frame transfer uses simpler technology (no photodiodes), and achieves higher fill factor
than interline transfer.
Interline transfer uses optimized photodiodes with better spectral response than the photo
gates used in frame transfer.
In interline transfer the image is captured at the same time (`snap shot' operation) and the
charge transfer is not subject to corruption by photo detection (can be avoided in frame
transfer using a mechanical shutter).
Frame transfer chip area (for the same number of pixels) can be larger than interline
Most of today’s CCD image sensors use interlines transfer.
Complementary Metal Oxide Semiconductor
“CMOS" refers to both a particular style
of digital circuitry design, and the family
of processes used to implement that
circuitry on integrated circuits (chips).
CMOS circuitry dissipates less power
when static, and is denser than other
implementations having the same
CMOS circuits use a combination of p-
type and n-type metal–oxide–
semiconductor field-effect transistors
(MOSFETs) to implement logic gates and
other digital circuits found in computers,
telecommunications equipment, etc.
Basic Operation of CMOS
The CMOS approach is more flexible
because each pixel can be read individually.
In a CMOS sensor, each pixel has its own
charge-to-voltage conversion, and the sensor
often also includes amplifiers, noise-
correction, and digitization circuits, so that
the chip outputs digital bits.
With each pixel doing its own conversion,
uniformity is lower. As shown above, the
CMOS image sensor consists of a large pixel
matrix that takes care of the registration of
The electrical voltages that this matrix
produces are buffered by column-amplifiers
and sent to the on-chip ADC.
Types of CMOS Image Sensors
1. Active Pixel Image Sensor 2. Passive Pixel Image Sensor
CCD vs CMOS
CMOS image sensors can incorporate other circuits on the same chip, eliminating the
many separate chips required for a CCD.
This also allows additional on-chip features to be added at little extra cost. These
features include image stabilization and image compression.
Not only does this make the camera smaller, lighter, and cheaper; it also requires less
power so batteries last longer.
CMOS image sensors can switch modes on the fly between still photography and video.
CMOS sensors excel in the capture of outdoor pictures on sunny days, they suffer in low
Their sensitivity to light is decreased because part of each photo site is covered with
circuitry that filters out noise and performs other functions.
The percentage of a pixel devoted to collecting light is called the pixel’s fill factor.
CCDs have a 100% fill factor but CMOS cameras have much less.
The lower the fill factor, the less sensitive the sensor is and the longer exposure
times must be. Too low a fill factor makes indoor photography without a flash
CMOS has more complex pixel and chip whereas CCD has a simple pixel and chip.
Application of Image Sensor
Personal Digital Assistance (PDA)
Toys and Robots
Finger print Scanner
Virtual Key Board
Image sensors applications in medicine
Security Market Volume of CCD, CMOS